It is highly documented that military personnel exposed to blasts from firearms, explosions and other high level peak noises are at high risk for hearing loss. Sound pressure levels (SPLs) over 160 dB that occur over periods as short as even a few milliseconds are sufficient to cause damage to the unprotected ear. Exposure to the peak noises caused by gunfire or other explosions adds hearing loss to the long list of risks and dangers encountered by soldiers on the battle field. One means to prevent hearing loss is to wear a noise attenuating device such as ear plugs or earmuffs. U.S. Pat. No. 5,203,352 issued to Gardner presents high-attenuation foam earplugs which may provide up to 40 dB of attenuation when properly inserted. Accordingly, the Gardner earplugs will reduce hazardous external peak SPLs of 160 to 190 dB to safer levels of 120 to 150 dB, respectively, within the ear canal of the wearer.
The Gardner and other similar earplugs will attenuate up to 40 dB of noise, but the attenuation level is independent of the level external sound. In other words, all external noises will be attenuated the same amount whether the sounds are extremely loud or very soft. Thus, softer sounds that would otherwise be audible without the use of earplugs may become inaudible or become so soft that they go unnoticed. For many work environments the perception of soft sounds is vital to the task at hand or the safety of the workers. For example, a soldier wearing earplugs as described attenuating a constant 40 dB of noise may fail to hear an enemy quietly approaching or fail to perceive communications from fellow soldiers. Likewise, a construction worker wearing such earplugs may receive adequate protection from high level construction sounds, but fail to hear a distant coworker's emergency call for help.
Many earplugs, like those described by Gardner, for example, may distort the reception of normal sound. The earplugs attenuate higher frequency sounds at a higher level than lower frequency sounds making it difficult for the wearer to hear or understand speech and other important sounds. High-audibility earplugs such as those described by U.S. Pat. No. 4,807,612 issued to Carlson, U.S. Pat. No. 5,113,967 issued to Killion et al., and U.S. Pat. No. 4,852,683 issued to Killion and products such as the ER-15® and ER-20® series earplugs produced by Etymotic Research, Inc.® produce relatively uniform attenuation across audible frequency ranges and a low enough attenuation such that speech and music remain highly audible to the wearer. While the high audibility of these earplugs allows the wearer to hear softer noises, they may not provide adequate protection for extremely high level sounds. A soldier wearing these ear plugs may still be able to hear a quietly approaching enemy, but the soldier's ears will remain exposed to dangerous noise levels that occur during battle.
An improved earplug with sound level dependent attenuation is described in U.S. Pat. No. 4,924,502 issued to Allen, et al. and U.S. Pat. No. 5,936,208 issued to Hamery and is embodied in a product sold by AEARO Technologies under the trade name Combat Arms Earplug. The Combat Arms Earplug introduces less noise attenuation for external SPLs below 110 dB than for external SPLs above 110 dB, but does not provide a constant attenuation across all frequencies. For example, where the external SPLs is below 110 dB the Combat Arms Earplug provides around 5 dB of attenuation at low frequencies and up to 20 dB attenuation at higher frequencies. Where the external SPL is above 110 dB, the attenuation provided increases by approximately 0.5 dB for each 1 dB increase of external sound until a maximum level of attenuation is achieved. The Combat Arms Earplug provides a passageway that allows partially unobstructed travel of sound from the exterior into the ear canal with a low level of attenuation at low sound pressure levels. A sharp obstruction located within the passageway of the Combat Arms Earplug causes the flow of sound within the earplug to become turbulent above 110 dB. This introduction of turbulent flow impedes the flow of the sound into the ear, thereby establishing greater attenuation. While the aforementioned earplugs may provide non linear pattern attenuation, they do not provide the high levels of sound attenuation necessary to adequately protect against high-level noises. Nor do they provide a constant attenuation level across all frequencies.
Thus, there exists a need for non linear attenuation earplug that provides a low, uniform attenuation at all frequencies and SPLs for external SPLs below a certain value (e.g. 110 dB SPL), yet provides a higher and increasing level of attenuation for external SPLs above that certain value.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application.